Does Diffuse Circumnuclear Gas around Sgr A* Achieve Collisional Ionization Equilibrium or Remain Non-equilibrium Ionization?

The feeding and feedback processes at the vicinity of a supermassive black hole (BH) are essential for our understanding of the connection between supermassive BH and its host galaxy. In this work, we provide a detailed investigation, both observational and theoretical, on the diffuse (∼2″–20″, ∼0.08–0.8 pc) X-ray emission around Sgr A*. Over two-decade Chandra observations are gathered to obtain highest signal-to-noise to date. We find that, the line center of iron lines of the outer 8″–18″ region, ϵ c = 6.65 − 0.03 + 0.02 keV , is comparable to that ( ϵ c = 6.60 − 0.03 + 0.05 keV ) of the inner 2″–5″ region. This is somewhat unexpected, since the gas temperature decreases further away from the central BH. Based on a dynamical inflow–outflow model that considers the gas feeding by stellar winds from Wolf–Rayet stars, we calculate the X-ray spectrum based on both the conventional collisional ionization equilibrium (CIE) assumption, and the newly developed non-equilibrium ionization (NEI) assumption. We find that, theoretically gases within ∼8″–10″ remain in a CIE state, outside of this radius they will be in the NEI state. A comparison of the properties of ∼6.6 keV iron lines between CIE and NEI is addressed. Interestingly, the NEI interpretation of outer region is supported by the Chandra line center ϵ c measurements of this region.